Question Statement:
A rain gauge station $X$ was inoperative during a storm. The normal annual precipitation at $X$ is $N_X = 1000\text{ mm}$, and at three surrounding index stations $A, B, C$ are $N_A = 900\text{ mm}, N_B = 1050\text{ mm}, N_C = 1100\text{ mm}$. The storm rainfalls recorded at $A, B, C$ were $P_A = 45\text{ mm}, P_B = 52.5\text{ mm}, P_C = 55\text{ mm}$. Using the normal ratio method, the missing storm precipitation at $X$ in mm is: